This change provides the Content Decryption Module (CDM) a way to verify
the host files, the CDM adapter and the CDM itself.

GetCdmHostFilePath() provides a list of host files.

CdmHostFile opens the pair of a file and its corresponding signature
file. CdmHostFiles manages all files (and signature files) opened for
verification.

On platforms that use the Zygote, files are opened in the Zygote process.
After a child process is forked, if it's not a ppapi process, all files
will be immediately closed.

On other platforms, files are opened after the ppapi process is
launched, but before the sandbox is sealed.

On all platforms, if the ppapi process hosts a CDM, VerifyHostFiles()
will be called with the files managed in CdmHostFiles. Then all opened
files will be closed.

A browser test is added to make sure files are opened correctly and are
passed in VerifyHostFiles() as expected.

BUG=658036
TEST=New browser_tests added.

Review-Url: https://codereview.chromium.org/2582463003
Cr-Commit-Position: refs/heads/master@{#446260}